With conventional light sources, light emitted by the source usually has a lower energy level. In optical lithography or metrology an excimer laser, for example, may be used as a light source, and the radiation output from the laser may be passed through a radiation beam-scrambling illuminator to distribute the light intensity, for example, uniformly over an area. The area may be a spatial light modulation (SLM) area. When using higher power laser beam sources, the energy may heat conventional (e.g., simpler) blocking apertures, which may result in a change in the dimensions and/or form of the blocking aperture. In an alternative, material from a conventional aperture stop may be re-deposited onto other components of the optical system on which the aperture stop may be arranged.